ORGANIC EVOLUTION 



243 



Convergence. — Convergence of development is less com- 

 mon than divergence, probably because divergent lines, 

 radiating outward from a common starting point, are more 

 likely to enter vacant territory, and thus avoid the stress 

 of competition. 



Convergence is manifest in the superficial resemblance 

 of forms that are in essential characters widely different. A 

 familiar example is furnished by the swift and the swallow 

 (fig. 150), birds so similar in appearance and habit as to be 

 readily confused by a novice; indeed, they were long 

 classified together by ornithologists. But they differ in 

 nearly every essential character, and are members of differ- 

 ent orders of birds. A comparison of their feet will reveal 

 some of the more obvious external differences. Those of the 

 swallow are of the song-bird type of covering; a series of 

 overlapping scales down the front of the "tarsus" and a 

 single, sharp-edged plate behind. (Compare the tarsus of the 

 catbird figure 147c.) And the toes, counting from the hind 

 toe outward are successively 2-, 3-, 4- and 5-jointed. The 

 tarsus of the swift is "rather skinny than scaly," and the 

 toes, taken in the same order, are 2-, 3-, 3- and 3 -jointed. 

 Moreover, the tail feathers of the swift are spiny tipped, as 

 in the woodpeckers, with which it has more affinity than 

 with the song birds. 



Study J J. A comparison of convergent species. 



Materials: It is perhaps inadvisable to specify particular 

 illustrative material here, since any teacher may have his 

 own "best illustrations" of this phenomenon, which he will 

 regard as most available. The following good examples, 

 will, however, be found readily procurable almost any^vhere: 

 i) a bird and a bat, to be compared for the parallel develop- 

 ment of organs of flight. 2) Two limpet-shaped insect larvae 

 common in rapid streams, the "water penny," (larva of a 



